1. Field of the Invention
This invention relates generally to a fuel-air mixture control apparatus for adjusting the mixture of fuel and air that a carburetor provides to an internal combustion engine.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Known carburetors generally include fuel-air mixture control apparati. A mixture control apparatus of this type will typically include a mixing passage that provides a fuel-air mixture to one or more combustion chambers of an internal combustion engine. Carburetors also generally include throttle valve plates and choke valve plates pivotally supported in their mixing passages to control the flow rate of the fuel-air mixture through the mixing passages. Main fuel supply passages admit fuel into the mixing passages through main fuel nozzles. Some carburetors also include an idle fuel supply passage that admits fuel to a mixing passage through an idle fuel nozzle to provide sufficient fuel to support idle operation of an internal combustion engine. Idle fuel ratio control valves in the form of valve needles are typically used to adjust the proportion of fuel drawn into the mixing passage to the flow rate of air drawn into the mixing passage.
It is known for small internal-combustion appliance engines to have carburetors designed for operation at only two throttle settings: idle and wide open throttle. For this reason, and because emissions from such engines are tested only at the wide-open throttle setting, it is desirable to provide means for preventing changes in idle fuel-air mixture setting from changing the fuel-air mixture setting at wide-open throttle.
For example, a line of carburetors manufactured by the assignee of the present invention and designated WT-283C carburetors, include a fuel-air mixture control apparatus that shuts-off idle fuel flow at wide-open throttle. As is schematically shown in FIG. 9 of the drawings, the apparatus includes a mixing passage 1 formed in a carburetor body 2 and a throttle valve plate 3 pivotally supported in the mixing passage 1. Main and idle fuel supply passages 4, 5 are formed in the carburetor body 2 and provide fuel to the mixing passage 1 through respective main and idle fuel nozzles 6, 7. An idle fuel ratio control valve 8 disposed in the carburetor body 2 adjusts the proportion of fuel drawn into the mixing passage 1 to the flow rate of air drawn into the mixing passage 1 at idle engine operation. The idle fuel supply passage 5 is in fluid communication with the main fuel nozzle 6 such that vacuum pressure induced by air flow rates across the main fuel nozzle 6 at or approaching wide-open throttle, precludes idle fuel flow into the mixing passage 1 through the main fuel nozzle 6. Consequently, the position of the idle fuel flow ratio control valve 8 cannot affect the fuel air mixture at throttle settings at or approaching wide-open. Because the throttle 3 must be positioned close to wide-open to produce sufficient vacuum pressure to halt idle fuel flow, idle mixture settings cannot affect the fuel-air mixture in the mixing passage 1 at the wide-open throttle settings. However, a fuel-air mixture control apparatus constructed according to WT-283C type carburetors does not provide for adjustment of the fuel to air ratio at throttle settings approaching wide-open. As such, changes in air density due to temperature changes (for example) cannot be compensated for at throttle settings approaching wide-open.
The invention is a fuel-air mixture control apparatus for adjusting the mixture of fuel and air that a carburetor provides to an internal combustion engine. The apparatus includes a carburetor body and a mixing passage preferably formed in the carburetor body. The mixing passage 1 is configured to provide a fuel-air mixture to one or more combustion chambers of an internal combustion engine. A main fuel supply passage provides fluid communication between a fuel source and the mixing passage 1 through a main fuel nozzle. An idle fuel supply passage provides fluid communication between a fuel source and the mixing passage 1 through an idle fuel nozzle. An idle fuel ratio control valve is configured to adjust the proportion of fuel drawn into the mixing passage 1 through the idle fuel supply passage to the flow rate of air drawn into the mixing passage 1. The idle fuel supply passage is in fluid communication with the main fuel nozzle and is configured such that vacuum pressure induced by air flow rates across the main fuel nozzle, which exceed a predetermined flow rate, precludes idle fuel flow into the mixing passage 1. A main fuel flow ratio control valve is configured to adjust the proportion of fuel drawn into the mixing passage 1 through the main fuel supply passage to the flow rate of air through the mixing passage 1.
Objects, features and advantages of this invention include providing a fuel-air mixture control apparatus that allows an operator to adjust the high speed mixture of fuel and air that a carburetor provides to an internal combustion engine at throttle settings approaching wide-open throttle while preventing idle fuel flow from affecting the high speed mixture, that allows an operator to select an optimum high speed fuel-air mixture setting without having to compensate for whatever idle fuel mixture setting was selected, that allows an operator to select any idle fuel mixture setting and then select an optimum main fuel-air mixture setting without having to compensate for whatever idle fuel mixture setting was selected; and that allows an operator to later change the idle fuel mixture setting without having to make a corresponding correction to the main fuel mixture setting to compensate for the change. Therefore, idle fuel flow has no effect on engine exhaust emissions at wide-open throttle settings.